Collection of objects added as particle to a Plane not getting projected within the Plane
The problem has to do with the way you created your letters. I'm guessing you added a Text object, converted to a mesh and then separated by loose parts. The problem with this is the whole text object is considered to have one origin point that stays the same for all the individual letters after they have been separated.
In the image below, you can see that no matter what letter object I pick, the origin point is still the same. Also notice how the offset of the particles corresponds somewhat to the offset of the each letter from the origin point.

The solution then, is to reset the origins for each letter back to the center of the object. Select a letter and pick Object &gt; Set Origin &gt; Origin To Geometry. Do this for each letter. To make it faster, you can do it for one letter, and repeat for the rest, by selecting them one by one and pressing Shift + R.

If you want to make sure the origin is at the bottom of the letters (so the particles don't appear &quot;halfway through&quot; the plane), you can be a bit more specific and (in edit mode) select a bottom face (or combination of faces) and press Shift+S and select &quot;Cursor to Selected&quot;. This will place the 3D cursor in the middle of the face(s) you selected.

From here, go back to Object mode, and select  Object &gt; Set Origin &gt; Origin To 3D Cursor. This is a more involved process though, as you will have to do this manually for each letter.